The present invention relates to an active-matrix type liquid crystal device or thin film integrated circuit devices using a semiconductor layer with crystallinity.
Active-matrix type liquid crystal devices have been well known which utilize thin film transistors (generally called as TFT). In a conventional active-matrix liquid crystal display device, a peripheral circuit member is constituted by IC and is externally connected to terminals of matrix wirings of the pixels. Further; it is also known to form TFTs for forming a peripheral circuit on the same substrate on which TFTs are arranged in a matrix as a switching element in a pixel region.
Since the TFTs formed on a peripheral circuit portion are to drive the TFTs formed in a matrix form on a pixel portion, they are required to have a capability of passing a large amount of electric current therethrough. Specifically, they are required to have a large ON current and a large mobility.
On the other hand, the TFTs formed on the pixel portion are not required to have a large mobility. Rather, they need to have a lower OFF current (leak current) in order to maintain electric charges on pixel electrodes. Accordingly, the required characteristics for the TFTs of a peripheral circuit are different from those for the TFTs of a pixel portion.
Further, an amorphous silicon film has been used to form a TFT but its characteristics are not satisfactory. Therefore, a TFT using a crystalline silicon film has been investigated. Generally, it is necessary to perform a thermal annealing at 600xc2x0 C. or higher and for more than 24 hours for obtain a crystalline silicon film from an amorphous silicon film. However, a glass substrate which is usually used as a substrate of a liquid crystal device can not endure such a thermal annealing because for example, a Corning 7059 glass has a distortion point of 593xc2x0 C. In particular, one problem is that the glass substrate tends to be distorted because of the high temperature so that it is difficult to increase the size of the substrate.
The inventors of the present application have confirmed through their experiments that by contacting a slight amount of catalyst metal such as nickel or platinum with an amorphous silicon film, it is possible to crystallize the silicon film at lower temperatures, for example, at 550xc2x0 C. for about 4 hours. And the resultant crystallinity is comparable with that obtained the above conventional thermal treatment at 600xc2x0 C. The inventors considered that these metal functions as a catalyst to promote the crystallization of an amorphous silicon film.
The inventors also confirmed that there are two types of crystallization in the case of using a catalyst as indicated below.
(1) A crystallization proceeds in a direction normal to a substrate in a region where a catalyst was introduced.
(2) A crystallization proceeds in a direction parallel with a substrate from a region to which a catalyst was introduced toward a region to which a catalyst was not introduced.
The crystal structure in the case of (2) was confirmed by using a TEM (transmission type electron microscope) that columnar crystals grow in a direction parallel with a substrate. Also, the amount of nickel necessary for causing the above first type of crystallization is different from the amount of nickel necessary for causing the above second type of crystallization. For example, when the (2) type of crystallization extends about 30 xcexcm, the amount of the nickel necessary to be introduced thereto is 10 times as much as that required in the case of (1).
In the present specification, hereinbelow, the region where the above (1) type of crystallization occurs will be called as a vertical growth region and the region where the above (2) type of crystallization occurs will be called as a lateral growth region.
It is an object of the present invention to form thin film transistors on a pixel region (pixel TFTs) and thin film transistors on a peripheral circuit region on a same substrate through a simple process for an active-matrix type liquid crystal device.
Moreover, it is another object of the present invention that, in an active-matrix type liquid crystal device, pixel TFTs have a different crystallinity than TFTs formed on a peripheral circuit region.
In accordance with one aspect of the present invention, the method of the present invention includes a step of irradiating a laser light or a light having a sufficient strength on a selected portion of a semiconductor film to constitute a peripheral circuit region.
In accordance with another aspect of the invention, TFTs constituting a peripheral circuit are formed with a crystalline silicon film in which crystals grew in a direction approximately parallel with a direction along which carriers of the TFTs flow while pixel TFTs are formed with a crystalline silicon film in which crystals grew in a direction approximately perpendicularly with respect to a carrier direction of the pixel TFTs. The carrier direction means the direction along source and drain regions of the TFT. Namely, when a carrier direction is approximately aligned with a crystal growth direction, since carriers move along the direction of grain boundaries, the influence of the grain boundaries with respect to the carrier flow is suppressed and thus such a TFT can have a higher mobility. Accordingly, such a TFT is suitable for forming a peripheral circuit which requires a higher ON current. On the other hand, when a carrier direction is perpendicular to a crystal growth direction, since carriers have to cross grain boundaries, the OFF current can be decreased while the ON current is decreased. Accordingly, such a TFT is suitable as a pixel TFT for switching a pixel electrode in an electro-optical device.
It is still a further aspect of the present invention that TFTs having a channel region formed with a vertical growth silicon film (i.e. in which crystals grew perpendicularly to the substrate) and TFTs having a channel region formed with a lateral growth silicon (i.e. in which crystals grew horizontally with the substrate) are both formed on the same substrate. Specifically, the TFTs associated with the vertical growth region is suitable as pixel TFTs for switching pixel electrodes while the TFTs associated with the lateral growth region is suitable as a peripheral circuit for driving the pixel TFTs. This feature is based on the inventors"" discovery that the amount of a catalyst required for forming a vertical growth film and that for forming a lateral growth film are different from each other. In order to accurately control the amount of a catalyst to be introduced into a semiconductor film, the inventors utilize a liquid for carrying a catalyst therein.
In accordance with the present invention the catalyst may be one or more elements selected from the group consisting of Ni, Pd, Pt, Cu, Ag, Au, In, Sn, P, As and Sb. Alternatively, the catalyst may be one or more elements selected from the group consisting of Group VIII elements, Group IIIb elements, Group IVb elements and Group Vb elements.